Within the framework of the development of nuclear power stations, it is necessary to employ safety valves which enable decompression of enclosures containing a fluid under pressure, in the event of the pressure of the fluid becoming higher than a limiting value fixed at the time of the design of the components of the power station.
This enclosure may contain a fluid in circulation and form a circuit such as the primary circuit of the reactor containing water under high pressure and at very high temperature, or it may form an enclosure containing a fluid under static pressure, whether this fluid be a liquid such as water under pressure or a gas such as steam.
In any case, these circuits or enclosures must be equipped with safety valves for the case where the pressure comes to exceed the permitted limits defined at the time of the design of the circuit or enclosure, taking into account a factor of safety.
These valves are intended for decompressing the circuit or enclosure very rapidly in the event of excess pressure.
Such valves consist of a hollow body mounted on the enclosure and communicating with an opening in it. Inside this hollow body a stem is mounted for movement, for the displacement of a valve gate attached to one end of it, between a position of closure in which the valve gate rests against a seat surrounding the opening in the enclosure and an open position in which the valve gate is removed from its seat. A calibrated spring bearing against the valve body on the one hand and against the valve stem on the other, enables a force to be exerted for returning the valve gate onto its seat, so that the valve gate remains in the closed position as long as the pressure in the circuit or enclosure does not exceed the nominal value.
At the time of lifting of the valve gate, a portion of the fluid under pressure is ejected outside the enclosure, which enables a pressure to be re-established which is less than the nominal value, the calibrated spring then enabling the valve gate to be brought back into the position of closure against its seat.
However, in the case of a gaseous or liquid fluid such as steam or superheated water or cold water, pressure waves are created upstream of the valve at the time of its opening, which may bring about unstable operation of this valve, i.e., high-frequency pulsations.
This working condition of unstable operation brings about serious disadvantages such as seizing of the valve or hammering in the steam pipework. This is so in particular when the valve is connected to a volume under pressure by pipework of a certain length.
When the valve must permit the escape of a non-compressible fluid, such as the water in the primary circuit of a pressurized-water nuclear reactor, towards a reservoir provided for this purpose, the fluid is set in motion instantaneously just upstream of the valve, whereas it remains stationary through inertia in the pipework at a certain distance from this valve. This causes a very sharp depression which has a tendency to reclose the valve immediately after its opening.
Further, in the case where the fluid is non-compressible, there is no expansion of it to enable the valve gate to be held in the open position above its seat. Hence there is a serious risk of unstable operation, pulsation of the valve and untimely closure.
A device for elimination of vibrations of a valve for decompression of an enclosure containing a fluid under pressure, at the time of its actuation under the effect of the pressure of the fluid, has therefore been proposed in French Pat. No. 81 20391. In this device, the valve comprises, in a hollow body mounted on the enclosure and communicating with an opening of the latter, a valve closure gate complemented by a seat encircling the opening, a stem integrally fixed to the gate, moveable in the body of the valve, for the opening and the closing of the gate, a spring bearing on the stem by one of its ends and on the valve body by its other end, to maintain the gate on its seat, as long as the pressure within the enclosure remains lower than a limiting pressure. The device for elimination of vibrations comprises an electromagnet arranged outside the valve body, having a fixed part comprising an exciting winding, and a moveable armature connected to the end of the valve stem opposite that carrying the gate. This armature is moveable between two stable positions corresponding respectively to the total or partial opening and to the closing of the gate. The stability of at least one of these positions is obtained by the magnetic force exerted by the winding on the moveable armature when the winding is energized. The device for elimination of vibrations also comprises a means of control of the energizing of the electromagnet, as a function of the value of the pressure in the enclosure relative to at least one reference pressure.
At the beginning of the raising movement of the valve stem, under the effect of a pressure within the enclosure greater than the reference pressure, the electromagnet being energized with electric current, the moveable armature clings to the end of the exciting winding, which prevents the valve stem and the gate from falling on the valve seat, even if the pressure falls within the enclosure or undergoes oscillations in the region of the valve seat.
Preferably, the valve stem is connected to the moveable armature through the intermediary of a lever which is articulated to the valve body comprising a part arranged within this valve body and a part arranged outside.
The part arranged within the valve body is connected to the stem while the end of the part arranged outside the valve body is connected to the moveable armature.
Such levers exist generally on the safety valves to which the device according to the French patent is applicable.
When the clinging of the moveable armature is produced, at the beginning of the rising movement of the safety valve, the movement of the valve stem can nevertheless continue, in the opening direction, to increase the section of the outflow of the fluid under pressure.
The connection between the valve stem and the lever must therefore allow a certain relative movement of these two components.
For example, a spring which deforms by compression during the relative movement may be interposed between the two components.
Such a device does not, however, permit the damping of the possible vibrations of the gate and of the valve stem during the open movement following the magnetic clinging of the moveable armature. Such vibrations can occur in the event of pressure fluctuations in the enclosure or in the region of the outlet opening of the latter. Their effects are however greatly attenuated since the gate cannot fall back to the closed position on its seat, the fall of the stem being prevented by the moveable armature and the lever being held in position by magnetic force.
It remains desirable nevertheless to avoid the vibrations equally during the opening movement of the valve following the magnetic clinging of the moveable armature.